An insert for a radiant tube which consists essentially of ceramic material with a specified rate of thermal expansion and a specified thermal conductivity, wherein the insert is the shape of a helix with a specified number of turns per unit of length and per unit diameter and a specified number of cross-sectional wings.
Many industrial process furnaces require special atmospheres and, thus, cannot be directly heated by means of gas combustion. These special atmosphere furnaces are often heated by means of a system in which gas-air combustion takes place within long metal alloy tubes which exit to the outside of the furnace wall to prevent contamination of the furnace""s atmosphere. These furnaces are primarily heated by radiation coming off of the tubes; thus these tubes are called xe2x80x9cradiant tubes.xe2x80x9d
Such xe2x80x9cradiant tubesxe2x80x9d are well known to those skilled in the art and are described, e.g., in applicant""s U.S. Pat. Nos. 5,655,599, 5,071,685, and 4,789,506; the disclosure of each of these United States patents is hereby incorporated by reference into this specification.
The radiant tubes sometimes contain xe2x80x9cinsertsxe2x80x9d to increase heat transfer from the combustion gases to the inside surface of the radiant tube. Thus, by way of illustration, U.S. Pat. No. 4,869,230 describes a xe2x80x9cturbulator insertxe2x80x9d in a radiant tube which is formed as a corrugated strip of metal alloy material (such as nickel-chromium alloy or iron-nickel-chromium alloy material) twisted to form a helix; the entire disclosure of this United States patent is hereby incorporated by reference into this specification.
Such metal alloy material inserts, while initially effective, are not very durable. Thus, some work has been done to replace these metal alloy material inserts with ceramic inserts. Illustrative of such work is the xe2x80x9cFLAME BUSTERxe2x80x9d ceramic inserts sold by Ipsen Industries, Inc. of Rockford, Ill. in the 1950""s. These inserts met with only limited commercial success, primarily because they would tend to break while in use and often damage the burner assemblies. By way of further illustration, reference may be had to the heat exchanger baffle system disclosed in U.S. Pat. No. 2,861,596 of Ipsen, the entire disclosure of which is hereby incorporated by reference into this specification; it is believed that the insert described in this patent is similar to such xe2x80x9cFLAME BUSTERxe2x80x9d insert.
These ceramic inserts were discussed in U.S. Pat. No. 4,153,035 of Carl-Heinz Stiasny, wherein it is disclosed that xe2x80x9cSuch flame breakers may be made of ceramic material. With a design of this type, heat losses . . . may be relatively high . . . . Furthermore, flame breakers of this type . . . involve the risk of breakage particularly where vibrations in the furnace occur . . . xe2x80x9d
The Stiasny patent issued in 1979. Since that time, to the best of applicant""s knowledge, no flame breaker has been described in the prior art which is made of ceramic material, which has a substantially helical shape, and which provides efficient heat transfer and consequent low energy consumption and, additionally, is durable.
It is an object of this invention to provide a flame breaker with a substantially helical shape which consists essentially of ceramic material, and which has a thermal expansion of less than 6xc3x9710xe2x88x926 meter/meter/degree Celsius, which has a thermal conductivity of greater than 30 watts/meters/degree Celsius.